The present invention concerns a stream generator, in particular a steam generator used in connection with the mechanical preparation of pulp, for generating pure steam required in a subsequent process, e.g., in the drying of paper in a paper machine, by utilizing the condensation heat of contaminated steam coming from a preceding process, e.g., a grinding process.
When wood chips are being ground by means of the so-called thermomechanical process (TMP) of prior art, wherein the wood chips are most usually introduced into a 1-phase or 2-phase system of a disc grinder, electrical energy is typically consumed at a rate of about 1.7 to 2.5 MWh per ton of ready paper pulp.
In such processes, whose efficiency is, according to literature references, 0.1 to 0.2%, almost all of the rest of the energy is recovered out of the process in the form of steam. In an integrated paper mill, this steam can be converted in a heat exchanger into pure steam and be used in the paper machine for drying the paper. The steam obtained in this way represents 50 to 75% of the steam consumed by a paper machine.
Such a process is now already in operation in many places around the world. A problem that remains is that, out of a TMP plant, quite varying quantities of steam are obtained to the paper machine, because these quantities depend on the degree of utilization of the grinders. In such a situation, if, for example, a grinding line, whose output may be of the order of 10 to 15 MW, falls off, a corresponding quantity of steam also becomes unavailable to the paper machine. In such a case, the power plant producing auxiliary steam must react to the altered situation rapidly.
Today, in many parts of the world, the costs of electricity and fuel oil are rather close to each other and, moreover, often the marginal cost of electricity is still closer to the cost of the heat equivalent of fuel oil than the average cost of electricity.
When a large grinding line is being started, said line having, e.g., two grinders in series and having a total output of the order of 10 to 15 MW, the grinders are loaded evenly while increasing the output continuously over 2 to 20 minutes, whereby steam, which is generated correspondingly, is obtained in proportion to the loading.
It is also possible to combine a steam generator in which contaminated TMP steam is converted into pure steam with an electrical boiler which compensates for the output of grinders falling off. Such a boiler goes on with the same total electricity load while using part of its power for producing pulp and part for direct steam generation in the steam generator.
In this procedure as well, there remains the problem that power cannot be shifted suddenly from the grinders to electricity-consuming and steam-producing electrodes without causing a violent fluctuation in the electricity supply network.
Now it has been noticed that, in such a steam converter, in which part of the power is produced by means of power electrodes, the above-discussed difficulties can be overcome surprisingly easily in the way to be indicated below.
The invention is based on the idea that the overall generation of steam is equalized by controlling the moistening area of the electrodes in the electrical boiler. More specifically, the steam generator in accordance with the present invention is characterized in that it comprises
a steam converter; PA1 an electrical boiler connected in parallel with the steam converter; PA1 at least one power electrode arranged within the electrical boiler and comprising an outer face; PA1 means for moistening the outer face of said at least one electrode; and PA1 means for controlling the moistening area of the outer face of said at least one electrode.